Fig. 28.6 ). First, they propel food from the mouth to the stomach. Second, the sphincters protect the airway during swallowing and protect the esophagus from acidic gastric secretions.
The stimuli that initiate the changes in smooth muscle activity that result in these propulsive and protective functions are mechanical and consist of pharyngeal stimulation during swallowing and distention of the esophageal wall itself. The pathways are exclusively neural and involve both extrinsic and intrinsic reflexes. Mechanosensitive afferents in both the extrinsic (vagus) nerves and intrinsic neural pathways respond to esophageal distention. These pathways include activated reflex pathways via the brain-stem (extrinsic, vagus) or solely intrinsic pathways. The striated muscle is regulated from the nucleus ambiguus in the brainstem, and the smooth muscle is regulated by parasympathetic outflow via the vagus nerve. The changes in function resulting from mechanosensitive stimuli and activation of reflex pathways are peristalsis of striated and smooth muscle, relaxation of the LES, and relaxation of the proximal portion of the stomach.
Functional Anatomy of the Esophagus and Associated Structures
The esophagus, like the rest of the GI tract, has two muscle layers—circular and longitudinal—but the esophagus is one of two places in the gut where striated muscle occurs, the other being the external anal sphincter. The type of muscle (striated or smooth) in the esophagus varies along its length. The UES and LES are formed by thickening of striated or circular smooth muscle, respectively.
Motor Activity During the Esophageal Phase
The UES, esophagus, and LES act in a coordinated manner to propel material from the pharynx to the stomach. At the end of a swallow, a bolus passes through the UES, and the presence of the bolus, via stimulation of mechanoreceptors and reflex pathways, initiates a peristaltic wave (alternating contraction and relaxation of the muscle) along the esophagus that is called primary peristalsis (
Fig. 28.7 ). This wave moves down the esophagus slowly (3–5 cm/s). Distention of the esophagus by the moving bolus initiates another • Fig.28.6The esophagus and associated sphincters have multiple functions involved in movement of food from the mouth to the stomach and also in protection of the airway and esophagus. PropulsivefunctionsProtectiveeffectsFood transfer to esophagus Allows entry of food into esophagus Transports bolus from pharynx to stomach Allows entry of food into stomach Protects airway from swallowed material Protects airway from gastric reflux Clears material refluxed from stomach Protects esophagus from gastric reflux LES Esophagus UES Pharynx
CHAPTER 28 The Cephalic, Oral, and Esophageal Phases of the Integrated Response to a Meal • Fig. 28.7 Changes in pressure in the different regions of the pharynx, esophagus, and associated sphincters initiated during a swallow. The pressure trace is a diagrammatic representation from that obtained during manometry in an awake human. Stimulation of the pharynx by the presence of a bolus initiates a decrease in pressure (= opening) of the UES and a peristaltic wave of contraction along the esophagus. Stimulation of the pharynx also relaxes the smooth muscle of the LES to prepare for entry of food.
• Fig. 28.8 Swallowing in the form of pharyngeal stimulation induces neural reflex relaxation of the LES and the proximal part of the stomach to allow entry of food.